In semiconductor manufacturing, ion implantation has become a standard technique for introducing conductivity-altering impurities into semiconductor wafers to produce various semiconductor-based products. Such ion implantation is used to change the material properties of portions of a semiconductor substrate. Specifically, during such ion implantation, a desired impurity material is ionized in an ion source, generated ions are accelerated to form an ion beam of a prescribed energy, and the ion beam is directed at the surface of the semiconductor substrate. The energetic ions in the ion beam penetrate into the bulk of the semiconductor substrate and are embedded into a crystalline lattice of the semiconductor substrate to form a region of desired conductivity.
It is essential during ion implantation to maintain uniform dosage at the semiconductor substrate. One factor that can effect dosage uniformity is ion charge exchange interactions in a beamline of the ion beam that can alter actual and detectable ion beam current levels reaching the semiconductor substrate, making the ion beam current at the semiconductor substrate difficult to accurately measure. When charged ion particles collide with each other or with other particles, they can undergo ionization or neutralization, thereby gaining or losing a charge state. It is known in the art that fluctuations in vacuum pressure in and around the ion beam will increase the likelihood of these ion charge exchange interactions. Unfortunately, fluctuations in vacuum pressure are common during ion implantation because of out-gassing caused by the ion beam contacting photoresist and/or other coatings on the semiconductor substrate. Out-gassing is a process by which gas particles are released when the ion beam hits the semiconductor substrate coatings. The release of these gas particles temporarily increases the pressure along the beamline and can cause the gas particles to collide with the ions in the ion beam causing them to change charge state. For example, double charged P2+ ions may collide with a particle and neutralize to single charge P+ ions. The effect of this on the semiconductor substrate may vary depending on where in the beamline the charge exchange interactions occur.
When performing ion implantation with multiple charged ions, such as, for example ion beams of P2+, P3+, or P4+, the likelihood of charge exchange interactions increases. When an ion implantation process based on a P3+ ion beam becomes contaminated, such as with P2+ ions, the uniformity of the dosage on the semiconductor substrate may vary. Some of the P2+ ions may impinge on the semiconductor substrate while others may travel on trajectories away from the semiconductor substrate. This variation may be difficult to detect in real time and may result in reduced yields. Thus, in view of the foregoing, it would be desirable to provide a technique for detecting ion beam contamination during ion implantation to prevent losses due to poor process control and which overcomes some or all of the inadequacies and shortcomings of known systems.